Remove unused --split-product-types flag (close #105)

[lmath]

See https://github.com/snowplow-incubator/igluctl/issues/105

This change would also require

• [x] updating the docs which reference --split-product-types

[lmath]

I think it makes sense to update the docs even before the release as the --split-product-types flag already is not functional.

Previous state of the docs:

Now the reference to split product types is removed:

see this update live here ✅